In a recent article on AgileConnection, Allan Kelly wrote the following about the timing of writing acceptance criteria (acceptance tests) for user stories:
The Right Time to Define
I am frequently asked, “When should we write the acceptance criteria?” Sometimes programmers resist giving an effort estimate for a story unless they can see the ACs—sometimes detailed ACs, at that. However, there is little point in POs (and testers) spending time on ACs until stories are about to be scheduled. After all, if ACs take hours to write and the story is not scheduled, their time is wasted.
Also, if ACs are added but then the story doesn’t get scheduled for a year, by that time the story and the ACs may have changed. Because old criteria are in place, it can be easy to overlook these potentially important changes.